Recently, in consideration of environmental issues, vehicles using electric motors as driving sources, such as electric vehicles, hybrid vehicles and fuel cell vehicles are attracting attention. Such a vehicle is provided with a rechargeable power storage unit for supplying power to the electric motor and for converting kinetic energy to electric energy and storing the same at the time of regenerative braking.
In the vehicle having an electric motor as the driving source as described above, power supply of larger capacity is desirable in order to improve running performance such as acceleration characteristic and mileage per charge. As a method of increasing power supply capacity, provision of a plurality of power storage units has been proposed.
By way of example, U.S. Pat. No. 6,608,396 discloses an electrical motor power management system providing a desired high DC voltage level in a high-voltage vehicle traction system. The electrical motor power managements system includes: a plurality of power stages for providing DC power to at least one inverter, each stage including a battery and boost/buck DC-DC converter, and wired in parallel; and a controller controlling the plurality of power stages such that the batteries of the plurality of power stages are uniformly charged/discharged whereby the battery voltage to the at least one inverter is maintained by the plurality of power stages.
The power storage unit storing electric energy utilizing electro-chemical reaction has a maximum allowable power that can be charged and discharged (hereinafter referred to as the allowable power) defined in accordance with its state of charge. The allowable power represents a limit value for protecting each electric storage unit against excessive degradation.
On the other hand, in determining a driving force required in accordance with the state of running of the vehicle, the allowable power as such is not considered, and main focus has been on the optimization among the plurality of power storage units for sharing the driving force determined in accordance with the state of running. For instance, in the electrical motor power managements system of U.S. Pat. No. 6,608,396 mentioned above, what is disclosed is only a structure for actively distributing predetermined load power source as a whole.
Therefore, if a target value of driving force is set exceeding the allowable range of power supply or power regeneration of the plurality of power storage units as a whole, excessive degradation possibly occurs in each of the power storage units, or drivability (driving characteristic) possibly lowers.